Rotor-stator mixing apparatus especially for single screw extruder

ABSTRACT

A single screw extruder comprises a mixer associated with the screw, and comprising a rotor driven by the screw and a stator, the rotor and the stator each carrying mutually facing interengaging rings of teeth whereby the material is urged outwardly from the screw along a first tortuous mixing path, and then returned inwardly to the screw along a second tortuous mixing path, with the teeth extending axially, or generally so, with respect to the longitudinal axis of the screw. The invention also includes a mixer comprising a cylindrical stator chamber having opposed, radially extending faces provided with axially projecting, radially spaced-apart rings made up of alternating teeth and ridges, a rotor rotatably fitted within the stator and provided on its opposite side faces with axially projecting, radially spaced-apart rings of alternating teeth and ridges with the rotor and stator rings interengaging with both radial and axial clearance so as to define a tortuous material flow path.

FIELD OF THE INVENTION

The present invention relates in one aspect to a single screw extruderfor extruding combinations of materials such as thermoplastics polymers,rubbers, waxes and solid additives, and in another aspect to a mixer forsuch materials. The mixer could however be employed in the manufactureof inks, paints and other materials where one or more of the componentsis liquid at the room temperature.

BACKGROUND OF THE INVENTION

Single screw extruders are very widely used in the plastic industry forproducing compounds of rubber and thermoplastic polymers with solidadditives. They are simple to construct and therefore relativelyinexpensive; however that they have limited distributive and dispersivemixing capacity has been long recognised and is well documented (cf“Single Screw Mixing: Problems and Solutions” Martin Gale, a paperpresented at a RAPRA Technology Ltd. seminar Aug. 6, 1995).

Further background as to the mixing limitations of single screwextruders is given in an article in Plastics Additives and CompoundingAugust/September 1995 pages 21-23 entitled “New dispersive mixers basedon elongational flow” and the associated patent, U.S. Pat. No.5,932,159, published Aug. 3, 1999. This stresses the need for a varietyof dispersive forces including elongation flow and multiple passesthrough regions of high stress, conditions which are normally difficultto generate within a single screw machine.

There are many devices that can be used to improve the distributivemixing capacity of single screw extruders; however these devices offeronly marginal improvements in dispersive mixing. A good example of thisis the cavity transfer mixer (U.S. Pat. No. 4,419,014) where the processmelt is transferred repeatedly between cavities in a rotor and opposedcavities in the barrel wall. The rotary motion of the rotor means thatthe material is constantly subdivided and re-orientated. However thisdoes not generate a high shear rate since the walls of opposing cavitiesare quite widely separated. In practical devices this low shear ratealso limits the maximum cavity size since there is a tendency for themelt to stagnate.

Pins can also be used to improve mixing either protruding radially fromthe barrel or from the surface of a rotor or the screw itself. Whilstpins do generate chaotic flow, improving distributive mixing, they dolittle in the way of dispersive mixing since the pins do not moverelative to a complementary shear surface. In the case of pinnedbarrels, gaps in the helical flight sweep over the pins generate highsheer events and also allow significant re-circulation of the polymermelt improving distributive mixing. However the proportion of materialsubjected to high shear is quite small.

OBJECT OF INVENTION

A basic object of the invention is the provision of an improved singlescrew extruder, and to a mixer for use with such extruder.

Summary of a First Aspect of the Invention

According to a first aspect of the invention there is provided a singlescrew extruder comprising a drivable screw, with at least one flight,located within a static barrel so as to define an annular, material flowgap between the exterior of the screw and the interior of the barrel, amixer associated with the screw, whereby material passes from anupstream portion of the flow gap, into the mixer and is then eitherreturned to a downstream portion of the flow gap or is discharged, withthe mixer comprising a rotor driven by the screw and a stator, the rotorand the stator each carrying mutually facing interengaging rings ofteeth whereby the material is urged outwardly from the annular gap alonga first tortuous mixing path, and then returned inwardly along a secondtortuous mixing path, the teeth extending axially, or generally so, withrespect to the longitudinal axis of the screw.

SUMMARY OF A SECOND ASPECT OF THE INVENTION

According to a second aspect of the invention there is provided a mixerfor mixing solids with liquids, liquids and liquids e.g. polymer alloys,for use with other devices or combinations of devices capable of drivingthe rotor and introducing material in a fluid state into the mixer undersufficient pressure to cause the material to be mixed to flow throughthe mixer, the mixer comprising:

(i) a cylindrical stator chamber having opposed, radially extendingfaces provided with axially projecting, radially spaced-apart rings madeup of alternating teeth and ridges,

(ii) a rotor rotatably fitted within the stator and provided on itsopposite side faces with axially projecting, radially spaced-apart ringsof alternating teeth and ridges, and

(iii) the rotor and stator rings interengaging with both radial andaxial clearance so as to define a tortuous material flow path.

ADVANTAGES OF THE INVENTION

The extruder in accordance with the first aspect has been found toprovide significant improvement in the extruder performance and thequality of extruded product compared with prior art single screwextruders, whilst the mixer in accordance with the second aspect hasbeen found to be particularly advantageous and to provide a fundamentalimprovement in the mixing of solids and liquids—such as a liquidthermoplastics material and solid additive and improve the manufactureof polymer alloys. The rings on the stator may be as thin as possiblewhilst maintaining mechanical integrity since their only functions areto provide a barrier to melt flow and complementary shear surfaces tothe rotor. This arrangement limits possible melt stagnation in the gapsbetween stator teeth. In addition, the screw may serve as a main bearingfor the mixer, whilst because the teeth are concentric around thebarrel, there is no constraint on the length of the teeth that may beprovided.

PREFERRED, OR OPTIONAL, FEATURES OF THE EXTRUDER

The mixer is located intermediate the ends of the screw.

The mixer is located at the discharge end of the screw/extruder.

As the extruder will be used for extruding a range of materials, it isclear that a suite of mixers exhibiting differing geometrical propertiesto provide different mixing capabilities, is desirable for optimummixing. Thus, to permit reasonably expedient changing of a mixer, theextruder is provided with readily releasable means eg a pair ofreleasable flanges, within which the mixer is housed.

The internal diameter of stator chamber is larger than the internaldiameter of the extruder to which it is attached and the rotor isrelatively short.

PREFERRED, OR OPTIONAL, FEATURES OF THE MIXER

Clearly, the mixer can be used in combination with any apparatus capableof introducing the materials to be mixed under sufficient pressure tocause these materials to flow through the mixer. One such apparatus is asingle screw extruder. Thus it is necessary to provide the mixer with anentry aperture and an exit aperture for material feed under pressureinto, through, and out of, the exit aperture of the mixer. In the mixerthe materials undergo four actions namely (i) a radial movement underpressure suitably from a central feed port to an outlet port, (ii) anorbital movement involving division of the radially moving material intoportions some of which go one way while vicinal portions go the oppositeway and (iii) a shearing action (iv) an elongational deformation.

This mixer differs from that of a conventional extruderconfiguration—with a long thin screw and any ancillary mixers arecontained in a narrow cylinder, i.e. the primary internal barreldiameter—since the mixer has, in it is preferred configurations, a shortbroad rotor within a chamber with an internal diameter greater than thatof the barrel to which it is attached. Looking at this basic geometrytwo significant advantages become clear. Firstly the shortest pathlength through the mixer increases only linearly with rotor radiuswhilst the volume available for mixing increases with the square of thatradius. Secondly angular velocity rises linearly with the rotor radius.This means that the largest mixing volume coincides with the highestpotential shear rates.

By interengage is meant that the teeth of a rotor or stator ring alwaysextend into the valley defined between a pair of adjacent rings of thestator or rotor, whilst the ridges may or may not extend into thatvalley. Means are provided to drive angularly the rotor or the stator orboth so that there is relative movement between the two. Normally onlythe rotor will be driven to rotate about its axis.

In one form of the mixer, the rings are concentric to the axis of therotor. However they may also be arranged eccentrically which results ina cleaning action when the rings approach each other. The maximumeccentricity is limited to the separation between the rings forming thecomplementary valley on the complementary component.

The maximum combined height of a given ring and tooth at any point onthe surface of either the stator or rotor is limited by the separationof the stator and rotor. This separation can vary between 0.1 and 300mm, preferably 1 to 100 mm.

The combined height of the ridges and teeth can be varied by any amountwithin this limit either around the circumference or along a radialpath. The variation can be either continuous or discontinuous, i.e. thetransition can be slopped or stepped, but in the preferred form, theridges and teeth are uniform in height.

The thickness of both the teeth and ridges around their circumferencecan be varied but in the preferred form is uniform. This thickness canrange from 0.1 mm to 100 mm, preferably 1-30 mm.

Any tooth may combine any or all of these characteristics and thetransition between them may be continuous or discontinuous, i.e. thetransition may be slopped or stepped.

The stator, rotor, ridges and teeth may be made from any material thatis dimensionally stable at the operating temperature and under themechanical strains generated in operation. Such materials include steel,ceramics, rubber and plastics.

The ridges and teeth can be either permanently or removably attached tothe rotor and stator. Removable ridges and teeth may also be so attachedas to allow their repositioning and re-orientation. The stator whichdefines the chamber around the rotor may also be either permanently orreleasably assembled around the rotor.

Preferably the stator is defined by a pair of mutually facing cup shapedinserts which are clamped together opening-to-opening. When assembledinto a single screw extruder, such clamping would be between theextended flanges of the front and rear barrels. In this way the mixinggeometry of the device may be altered by replacing the relativelyinexpensive inserts.

In operation, the pressure gradient from inlet of the mixer to outletcauses material to flow through the device. There are three tortuousroutes that the material can take. A zigzag route over the intermeshingannular ridges and teeth, a route along the annular channels defined bythe ridges and a route through the gaps between teeth. All three routesare continuously changing due to relative motion and take material upone a face of the rotor across its edge and back down the obverse.

In one configuration the gaps between teeth on the stator and the rotorare arranged to form radial channels. The rotational motion of the rotorleads to periodic alignment of teeth and gaps and gaps and gaps betweenthe rotor and stator.

However the gaps may be staggered to alter the mixing characteristics.For any given ring the combined length of teeth and gaps is equal to thecircumference of that ridge. The teeth may be of any length along thering within this total but need not be uniform in length.

Dispersive mixing occurs in the gaps between the faces of the annularridges and teeth on the rotor and their counterparts on the stator.Material within these gaps is subject to both pressure and drag flow dueto the motion of the rotor. In this way the melt in the high shear zoneis constantly refreshed. Distributive mixing then ensures that thiswell-dispersed material is evenly distributed through the bulk of themelt.

Distributive mixing results from the repeated cutting of the melt as itemerges from gaps between teeth on the stator and also as it enters thenext set of gaps between the teeth on the stator. Since the speed offlow into and out of these gaps is slower near to the defining teeththan in the middle, significant reorientation of the melt also occurs.

The mixer can be used in conjunction with a number of functionalancillary elements each of which can be supported by a number of wellknown devices. The ancillary elements include:

a) means of receiving the process material;

b) means of melting one or more of the components to generate liquid;

c) means of degassing the process melt of the inlet side of the mixer;

d) means of generating pressure on the inlet side of the mixer;

e) means of driving the rotor of the mixer;

f) means of degassing the process melt on the outlet side of the mixer;

g) means of generating pressure on the outlet side of the mixer;

h) means of filtering the process melt; and

i) means of forming the process melt. Elements d and e are alwaysrequired for the operation of the mixer. The need for the otherancillary functional elements is dependent on the users needs, but asingle screw extruder can provide functional elements a, b, c, d and ethen f, g, h and i. Elements d and e are always required for theoperation of the mixer. The need for the other ancillary functionalelements is dependent on the users needs. Devices suitable for providingone or more of the functional ancillary elements include:

1) A single screw extruder can provide functional elements a, b, c, dand e then f, g, h and i

2) A twin screw extruder can provide functional elements a, b, c, d ande then f, g, h and i

3) A Z-blade mixer can provide functional elements a, b and c

4) An internal mixer can provide functional elements a, b and c

5) A gear or other pump can provide functional elements d, e and g

6) An external motor can provide functional element e

7) A removable filter system can provide functional element h

8) A removable die can provide functional element i.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described in greater detail, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 illustrates a preferred form of a single screw extruder inaccordance with the first aspect of the invention, incorporating a mixerin accordance with the second aspect of the invention;

FIG. 1a illustrates an enlarged view of the mixer shown in FIG. 1.

FIG. 1b shows a modification of the extruder of FIG. 1, in which themixer is located at the discharge end of the screw.

FIG. 2 is an end view of a front mixing insert (10);

FIG. 2a is a cross section of FIG. 2 along line 2 a— 2 a;

FIG. 2b is a cross section of FIG. 2 along line 2 b— 2 b;

FIG. 3 is an end view of mixing rotor (11);

FIG. 3a is a cross section of FIG. 3 along line 3 a— 3 a;

FIG. 3b is a cross section of FIG. 3 along line 3 b— 3 b;

FIG. 4 is an end view of an intermesh between front mixing insert (10);and mixing rotor (11);

FIG. 4a is a cross section of FIG. 4 along line 4 a— 4 a;

FIG. 4b is a cross section of FIG. 4 along line 4 b— 4 b;

FIG. 5 is an end view of an intermesh between front mixing insert (10),and mixing rotor (11) rotated 33.75 degrees clockwise in comparison toFIG. 4;

FIG. 5a is a cross section of FIG. 5 along line 5 a— 5 a;

FIG. 5b is a cross section of FIG. 5 along line 5 b— 5 b;

FIG. 6 is an end view of an alternative front mixing insert withoutradial alignment of the gaps between teeth;

FIG. 6a is a cross section of FIG. 6 along line 6 a— 6 a;

FIG. 6b is a cross section of FIG. 6 along line 6 b— 6 b;

FIG. 7 is an end view of an alternative mixing rotor;

FIG. 7a is a cross section of FIG. 7 along line 7 a— 7 a;

FIG. 7b is a cross section of FIG. 7 along line 7 b— 7 b;

FIGS. 8a to 8 e are cross sections of possible alignments of teethwithin the channels;.

FIG. 8a illustrates a tooth symmetrically aligned in the channel;

FIG. 8b illustrates a tooth aligned with the outside channel wall;

FIG. 8c illustrates a tooth aligned with the inside channel wall;

FIG. 8d illustrates a tooth with a negative pitch;

FIG. 8e illustrates a tooth with a positive pitch;

FIGS. 9a to 9 e are cross sections of possible variations in toothleading edges and tips with the arrow A showing the direction ofrotation

FIG. 9a illustrates a neutral leading edge with neutral tip;

FIG. 9b illustrates a positive leading edge and neutral tip;

FIG. 9c illustrates a negative leading edge and neutral tip;

FIG. 9d illustrates a neutral leading edge and positive tip rake;

FIG. 9e illustrates a neutral leading edge and negative tip rake;

FIGS. 10a to 10 c are cross sections of ridges and teeth illustratingpossible variations in leading edge shape;

FIG. 10a illustrates a symmetrical leading edge;

FIG. 10b illustrates a negative edge bias;

FIG. 10c illustrates a positive edge bias;

FIGS. 11a to 11 e illustrate possible variations in tooth cross-section;

FIG. 11a illustrates a parallel tooth in parallel channel;

FIG. 11b illustrates a negative sweep in parallel channel;

FIG. 11c illustrates a positive sweep in parallel channel;

FIG. 11d illustrates a tapered tooth in tapered channel;

FIG. 11e illustrates a inverted tapered tooth in inverted taperedchannel;

FIG. 12 illustrate possible variations in insert end wall geometry;

FIG. 12a is a section through 12 a— 12 a for a conical end wall;

FIG. 12b is a section through 12 a— 12 a for an inverted conical endwall;

FIG. 13 illustrates a possible variations in rotor disk geometry;

FIG. 13a is a section through 13 a— 13 a for an inverted conical disk;

FIG. 13b is a section through 13 a— 13 a. for a conical disk.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the single screw extruder comprises a spline 1 orother coupling from the rotary drive to rear extruder screw 4, a coolingjacket 2, a funnel and feed zone 3, a rear extruder screw 4, a rearbarrel 5, an enlarged flange 5A, a front barrel 6, an enlarged flange6A, a die 7, a discharge nozzle 8, a front screw 9, a rear mixing insert12 and a front mixing insert 10 which together form a chamber about amixing rotor 11 and a drive spindle 13. The mixer 14 is provided with aplurality of co-axial holes 5 b, 6 b to receive bolts (not shown) whichconstitute a readily releasable means to permit change of one mixer 14for another mixer, e.g., having different mixing characteristics. Thescrew of the single screw extruder is thus interrupted by the presenceof the mixer 14, comprising elements 10, 11, 12, and 13.

FIG. 1a is a drawing of one embodiment of the invention where the mixer14 consists of a rear mixing insert 12 and a front mixing insert 10which together form a chamber about a mixing rotor 11 and a drivespindle 13. Referring to FIG. 1b, barrels 5 and 6 can be heated and zone3 can be cooled by means of a cooling jacket 2. In addition the flangesjoining the extruder barrels 5 a and 6 a can be heated or cooled tomaintain a balance between heat losses to the environment and heatgenerated within the mixer again by means not shown.

Referring to FIGS. 2, 2 a and 2 b, the inside surface of insert 10 ismade up of a series of radially spaced-apart rings 22 a, 22 b, 22 c, and22 d. Mounted on the rings 22 a- 22 d are circumferentially spaced-apartand axially projecting teeth 21 so that gaps between adjacent teeth 21constitute ridges 20 giving the rings 22 a- 22 d a castellatedappearance.

Referring to FIGS. 3, 3 a and 3 b, the rotor 11 is mounted on a drivespindle 13 and consists of a circular body, opposed faces of which areeach provided with a series of radially spaced-apart rings 32 a, 32 b,32 c and 32 d. Mounted on the rings 32 a- 32 d are circumferentiallyspaced-apart and axially projecting teeth 31 so that gaps betweenadjacent teeth 31 constitute ridges 30 giving the rings a castellatedappearance. The rings 32 a, 32 b, 32 c and 32 d intermesh with rings 22a, 22 b, 22 c and 22 d in the functional position illustrated in FIGS.4, 4 a and 4 b and in FIGS. 5, 5 a and 5 b. The rings 22 a- 22 d, 32 a-32 d, teeth 21, 31, and ridges 20, 30 are so juxtaposed as to define atortuous, mixing flow path for material, with the arrows 40 and 41 (FIG.4) illustrating the path of least resistance to the melt flow throughridges (gaps) 20 between teeth 21. As drawn in FIGS. 4-4b and 5-5 b, theflow off the rotor 11 is not constrained by the second insert (12) i.e.only one half of the mixer 14 is shown.

As the rotor 11 moves in relation to its housing the following positionsarise:—(a) the teeth 31 on the rotor 11 are in line with the teeth 21 onthe ridges 20 (FIG. 4a), (b) the teeth 31 on the rotor 11 are cut out ofline with the teeth 21 on the ridges 20 (FIG. 4b) and (c) the teeth 31on the rotor 11 are in positions between (a) and (b). Position (a)represents the moment of maximum shear i.e. dispersion and maximumradial flow of the materials being mixed. Position (b) represents themoment of maximum distribution when aliquots of the outward flow ofmaterial are taken off in different directions according to whether theyare in locations bounded by rotor teeth 31 or locations bounded byhousing teeth 21. FIGS. 5, 5 a, and 5 b show the same elements, rotated33.75 degrees clockwise in comparison to FIG. 4.

FIGS. 6, 6 a, and 6 b show an insert 10 where the gaps 20 are notaligned to give a contiguous linear radial path and so the path of leastresistance involves annular as well as radial displacements.

FIGS. 7, 7 a, and 7 b show a rotor 11 where the teeth 31 are not alignedto give a contiguous linear radial path and so the path of leastresistance involves annular as well as radial displacements.

FIGS. 8a- 8 e, 9 a- 9 e and 10 a- 10 c show various possible toothgeometries. Referring to these figures it can be appreciated that theshape of the leading edge and the orientation of the individual teethcan promote movement of the melt in particular directions within themixer 14, which may be highly advantageous to both transport of materialthrough the mixer 14 and also in terms of promoting dispersiveconditions such as elongation flow.

FIGS. 8a- 8 e show a rotor tooth 82 between stator teeth 81 and 83 withthe tooth centred in FIG. 8a, scraping the outer tooth 81 in FIG. 8b,scraping the inner stator tooth 83 in FIG. 8c and shows a positive pitch(FIG. 8e) of the tooth 81 body which will push material outwards withinthe mixer 14 whilst a negative pitch (FIG. 8d) will bring it towards thecentre.

FIGS. 9a- 9 e show a tooth 31 leading edge having a neutral rake (FIG.9a), a positive rake on the leading edge (FIG. 9b) which will transportmaterial from the base of a valley towards the rotor 11 disc, whilst anegative rake (FIG. 9c) will push material down into the stator. Thearrow A indicates the direction of rotation. FIGS. 9d and 9 e show aneutral rake with variation in the configuration of the tooth 31 tip.

FIGS. 10a- 10 c show rotor tooth 101 between stator teeth 100 and 102with a positive bevel (FIG. 10c on the leading edge which will pushmaterial outwards within the mixer 14 whilst a negative bevel (FIG. 10bwill bring it towards the centre.

FIGS. 11a- 11 e illustrate that the configuration could be a paralleltooth 31 in a parallel channel 21 (FIG. 11a); a negative tooth 31 in aparallel channel 21 (FIG. 11b); a positive tooth 31 in a parallelchannel 21 (FIG. 11c); a taper tooth 31 in a taper channel 21 (FIG.11d); or an inverted taper tooth 31 in an inverted taper channel 21(FIG. 11e).

Alternative geometries are also possible for insert 10 and rotor 11. Asshown in FIGS. 2, 2 a, and 2 b and described above, the inside surfaceof insert 10 may be made up of a series of radially spaced-apart rings22 a, 22 b, 22 c, and 22 d. Mounted on the rings are circumferentiallyspaced-apart and axially projecting teeth 21 so that gaps betweenadjacent teeth constitute ridges 20 giving the rings a castellatedappearance. As shown in FIGS. 3, 3 a, and 3 b and described above, therotor 11 is mounted on a drive spindle 13 and consists of a circularbody, opposed faces of which are each provided with a series of radiallyspaced apart rings 32 a, 32 b, 32 c and 32 d. Mounted on the rings arecircumferentially spaced-apart and axially projecting teeth 31 so thatgaps between adjacent teeth constitute ridges 30 giving the rings acastellated appearance. Referring to FIGS. 12, 12 a, and 12 b,alternative embodiments are shown wherein insert 10 may comprise aconical (FIG. 12a) or an inverted conical (FIG. 12b) end wall. Referringto FIGS. 13, 13 a and 13 b, corresponding alternative embodiments forrotor 11 disk geometry are shown comprising an inverted conical disk(FIG. 13a) and a conical disk (FIG. 13b).

Use of the extruder of the present invention will now be described. Apolymer, polymer blend or polymer/solid additive mixture is fed into theextruder via the hopper and feed port 3 and is conveyed along the rearbarrel 5 by the rotation of the rear screw 4 which is driven by anexternal drive (not shown) via the coupling 1. Heat supplied via thebarrel 5 causes the polymer to become liquid or at least readilydeformable before reaching the rear mixing insert 12.

The mixing inserts 10, 12 could be integrated into the flanges 5A, 6A ofthe two barrel sections 5, 6 but by making them removable, mixinggeometry and hence mixing effects can be changed by replacing therelatively inexpensive inserts rather than the entire barrel.

The front 10 and rear 12 inserts are clamped together by flanges to forma cylindrical chamber with inlet 12 a and outlet 10 a as shown in FIG.1a. The surfaces of the mixing inserts 10, 12 and rotor 11 are coveredwith a series of annular ridges 20, 30. An example of this is shown inFIGS. 2 and 3. Ridges 20, 30 are offset from each other so that they canintermesh as shown in FIG. 4. On top of the ridges 20, 30 are a seriesof teeth 21, 31. The polymer melt can flow around the annular channels,outwards or inwards through the gaps 20, 30 between the teeth 21, 31 orfollow a longer path over the tips of the teeth 21, 31. Under operatingconditions the rotational motion of the rotor 11 results in a chaoticcombination of these. The net sum of all these routes takes the materialup one face of the disk across its edge and back down the obverse.

From the discharge port 10 a the polymer melt is conveyed by the frontscrew 9 along the front barrel and pressure is generated to force themelt through the discharge nozzle 8 in the die 7.

What is claimed is:
 1. A single screw extruder, for extrusion ofmaterial such as polymer, comprising a drivable screw having alongitudinal axis and a diameter, at least one flight provided on saidscrew, a static, parallel barrel rotatably housing said screw so as todefine an annular, material flow gap between an exterior of said screwand an interior of said barrel, a mixer to receive material delivered bysaid screw and through which mixer said material is forced, said mixercomprising a rotor driven by said screw and a stator, said rotor andsaid stator each carrying rings of teeth adapted to overlap, whereinsaid material, upon entering said mixer is firstly urged radiallyoutwardly along a first tortuous mixing path, and then returned inwardlyalong a second tortuous mixing path, said teeth extending axially, orgenerally so, with respect to said longitudinal axis of said screw,wherein: (i) said mixer has a diameter substantially exceeding that of adelivery end of said screw, such that said teeth of both said rotor andsaid stator of said mixer are outside said diameter of said screw, and(ii) the material is forced through said mixer principally by thepressure generated within said barrel by said screw.
 2. An extruder asclaimed in claim 1, wherein said mixer is located intermediate oppositeends of said screw.
 3. An extruder as claimed in claim 1, wherein saidmixer is located at a discharge end of said screw.
 4. An extruder asclaimed in claim 1, wherein readily releasable means are provided,permitting ready changes of one said mixer for another said mixer havingdifferent characteristics to suit changes of material being extruded. 5.An extruder as claimed in claim 1, wherein said rotor has a higheraspect ratio per unit axial length than said screw to which it isattached.
 6. An extruder as claimed in claim 1, wherein said statorteeth are provided on removable inserts.
 7. An extruder as claimed inclaim 1, wherein said rotor teeth are provided on a removable rotor. 8.An extruder as claimed in claim 1, wherein said mixer comprises saidrotor rotatable within said stator with opposed faces of said rotor, andopposed faces of said stator each having at least one circumferentialextending ring of interrupted, axially projecting teeth, wherein: (i)said stator comprises a chamber having said opposed side faces, whereina plurality of axially projecting, radially spaced-apart rings havingalternating teeth and ridges with channels defined between said adjacentrings are provided on each said opposed face of said chamber of saidstator, and (ii) said rotor has said opposed side faces, and a pluralityof axially projecting, radially spaced-apart rings incorporatingalternating teeth and ridges with channels defined between said adjacentrings are provided on each of said opposite side faces of said rotor,and (iii) said rotor rings interdigitate with said stator rings, withone of said stator rings located in one of said rotor channels and oneof said rotor rings located in one of said stator channels, such that atleast tips of said stator ring teeth overlap at least tops of said rotorring ridges, and at least tips of said rotor ring teeth overlap at leasttops of said stator ring ridges, with both radial and axial clearance soas to define, in the direction of material flow through said mixer, saidfirst tortuous mixing path flowing radially outwardly from the axis ofrotation of said rotor, followed by said second, tortuous mixing pathflowing radially inwardly to the axis of rotation of said rotor.